A listing of computer programs and computer listings for date patterns referenced in the specification are set forth in microfiche appendices comprising 258 frames on five (5) microfiche.
The present invention relates to a radar processing system and technique, and more particularly to a vector phase angle processing technique for detecting the spectral width of radar returns.
The analysis of weather conditions by radar processing techniques has been utilized for some time. It has been particularly important for providing weather information to aircraft pilots so that they may avoid flying in areas of rain, thunderstorms, or other adverse weather conditions. As a general rule, in addition to making a flight more uncomfortable, severe weather conditions pose potential hazards which may affect the safety of aircraft flight. Accordingly, pilots have used both on-board weather radars and ground radars for recognizing and avoiding flight areas that may be uncomfortable or unsafe, depending upon the severity of the weather.
One of the most troublesome and potentially dangerous weather conditions is that produced by turbulence. Turbulence may be present as a sheer zone in which there are variable velocity vertical air currents moving in opposite directions adjacent to one another. Turbulence may also be present in the form of air currents having stirring motions rather than the above-described sheer zone. In both cases, the continually changing velocities are of primary concern. It is the changing velocity of the air mass which provides significant hazard to aircraft flight, particularly in the case of sheer zones.
In the prior art, various weather radars, and in particular Doppler radars, have used a variety of techniques to enable turbulence detection. One such technique includes the visual display of the gradient of precipitation in a storm area as the radar monitors the particular rates of precipitation. In this instance, the gradient is viewed as a change in the rate of precipitation between adjacent areas. In cases where the gradient exhibits extreme changes, there is a likelihood that turbulence is present. Accordingly, the display of the gradient may be used as an indication of turbulence.
The above technique, however, has several disadvantages. First, a display of the gradient does not always indicate the presence of turbulence. Pilot interpretation of the gradient, for example, may be inaccurate, since turbulence may be present even in the absence of an extremely changing gradient. In other instances, turbulence may be absent even in the presence of an extremely changing gradient. In either case, pilot reliance on the gradient display can result in the operation of an aircraft to avoid turbulence that does not exist, or the operation of an aircraft in a turbulent area where turbulence is not visually indicated. In addition, prior art systems displaying the gradient are affected by the attenuation of radar returns due to intervening but undetectable precipitation. In those instances, the attenuation caused by the intervening precipitation can alter the magnitudes of the radar returns, thereby resulting in the display of inaccurate precipitation rates.
Other known systems also provide radar indications of turbulence, using a determination of spectral width. By way of example, pulse pair processing (PPP) and vector phase change (VPC) techniques, such as those described in the article "Spectral Mean and Variance Estimation via Pulse Pair Processing" by L. R. Novek, 16th Radar Meterology Conference, April 22-24, 1975, pp. 1-5, and "Estimation of Spectral Density Mean and Variance by Covariance Argument Techniques" by Dale Sirmans and Bill Bumgardener in the same 16th Radar Meterology Conference, April 22-24, pp. 6-13, have been employed to make determinations of spectral widths, as an indication of turbulence. In such systems, the Doppler radar transmitter transmits a pure frequency which is reflected by the target, in this case precipitation or other weather conditions. The reflected radar return, in contrast to the transmitted frequency, is not a pure frequency but a spectrum of frequencies determined by the velocity changes in the target being observed. It is known that in turbulent areas, the spectral width of the radar return increases over its width in the absence of turbulence. Accordingly, the PPP and VPC techniques are designed to make determinations of that spectral width as a measure of the turbulence present in a selected target area.
Although prior art PPP and VPC systems have been somewhat successful, various problems have been encountered in using the computed spectral width determinations to indicate turbulence. In particular, although such systems may be useful when employed in ground stations to determine weather conditions from a stationary position, the same systems are highly unreliable and inaccurate when used in aircraft due to aircraft movement, antenna scan, and other size, weight and cost restrictions. In particular, aircraft velocity changes often cause frequency shifts in the return which look like changes in the spectral width and thereby provide false indications of turbulence. Further, ground systems may employ large, expensive and powerful radar systems which cannot be used in an aircraft weather radar to enable in-flight pilot analysis. Additionally, prior art techniques often require substantial processing capabilities and hardware which prohibit implementation of the analysis by state-of-the-art digital processing techniques and microprocessor control.
In still other instances Fast Fourier Transform (FFT) chips have been used to provide the spectral analysis. However, in this type of system, an FFT chip is required for each range bin thereby leading to unacceptable hardware complexity. Additionally, inaccuracies caused by spectrum spread with antenna scan angle and complex threshold interpretations may limit the system to acceptable operation only when the antenna scan is stopped or slowed. Accordingly, in view of the trend toward complete digital implementation of aircraft avionics systems, there is a strong need for radar return processing systems which are capable of analyzing the returns on a real-time basis under microprocessor control to provide highly accurate measurements of spectral width as an indication of turbulence at normal antenna scan speeds.
Accordingly, the present system and technique has been developed to overcome the specific shortcomings of the above known and similar techniques, and to provide a spectral width detection system which allows real-time spectral width estimation of radar returns.